1. Field of the Invention
The present invention relates to a method of detecting a moving object from captured images.
2. Description of the Related Art
Space debris is an out-of-service artificial object which orbits the Earth. A detection of the existence of the space debris is important for smooth operation of spacecraft (artificial satellites, space stations, space shuttles and the like). One method among conventional space debris detection methods determines the existence of space debris by extracting high-luminance parts respectively from images captured by a CCD (charge coupled device) camera on the Earth or on the orbit.
This method includes: cropping areas in the same size from three or more images captured at time intervals in accordance with the movement of a moving object such as space debris; and applying a stacking method to the cropped image areas. Here, the movement of the moving object is the uniform motion or the like having mobile vectors (velocities) in a two-dimensional direction in the xy-coordinate system, for example. To put it concretely, the cropped image areas are superposed on one another, and a median value is evaluated from pixel values at the same pixels across the superposed image areas. If the median value thus evaluated is equal to or greater than a predetermined value, the pixel is extracted as a pixel representing the existence of the moving object (see Japanese Patent Application Laid-open Publications Nos. 2002-139319 and 2003-323625, for example).
The stacking method is a widely-used effective method not only for detecting space debris, but also for detecting a moving object at a low luminance level in an image. Furthermore, as described above, the stacking method employs the median pixel value as the evaluation value for the moving object detection. Therefore, the stacking method causes no rise in the evaluation value even if a noise component happens to be captured in some of the images (for example, a star or a cosmic ray in the cosmic space is captured in some of the images), in contrast to another case in which an average pixel value is used as the evaluation value. For this reason, the stacking method is advantageous in inhibiting erroneous detection of a moving object due to the existence of a star or a cosmic ray which happens to be captured therein.
Dark correction on an offset component unique to the CCD element, and flat correction on limb-darkening attributed to the characteristics of an optical system (a lens and a Newtonian reflector) of an astronomical telescope which is used in combination with the CCD camera are considered as being effective to enhance the detection accuracy as well (see Yanagisawa, T. and Nakajima, A. [2005]. “Automatic Detection Algorism for Small Moving Object.” Publications of the Astronomical Society of Japan, vol. 57, pp. 399-408, for example).
Furthermore, correction on a gradational change in brightness of the background light, which occurs due to clouds in the sky, scattering of light in the atmosphere, or existence of an area with a locally high luminance is also considered as being effective to enhance the detection accuracy (see Paolillo, F. and Porfilio, M. “First Italian Space Debris Observatory: Images Processing Automation.” Proceedings of International Astronautical Congress 2007, IAC-07-A6.I.05, for example).